Wenqiang Li,Aimin Liu,Yanying Jiang,Chengdan He,Fengguo Liu,Yongjun Wang,Xianwei Hu,Jin Wang,Zhongning Shi
Silicate minerals on S-type asteroids are a potential resource for in situ production of silicon. We investigated the feasibility of synthesizing silicon from Mg2Si3O8 through electrolysis in NaF-LiF molten salt. Solubility of Mg2Si3O8 in NaF-LiF molten salt was determined using isothermal saturation. Dissolution equilibrium is reached after 120 min, and the solubility of Mg2Si3O8 in NaF-LiF molten salt at 750 °C is 28.9%. Electrochemical studies revealed that the reduction of Si(IV) in NaF-LiF molten salt is a two-step process. Si(IV) was reduced to Si(II) at −1.1 V (vs Pt), and Si(II) was further reduced to metallic silicon at −1.2 V (vs Pt). Electrolysis of Mg2Si3O8 in NaF-LiF molten salt was conducted at 750 °C using a high-purity graphite sheet as anode and a silver sheet as cathode. Electrolysis products were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). XRD indicated that the main phase of the electrolysis products was silicon when cell voltages were 2.0, 2.5, 3.0, and 3.5 V. Raman spectroscopy confirmed the presence of crystal silicon. SEM and EDS revealed that the electrolysis products exhibit granular morphology, and silicon concentration reached a maximum value of 98.14% when the cell voltage was 2.5 V.
Goethitic bauxite is a widely used raw material in the alumina industry. It is an essential prerequisite to clarify the effect of Ti- and Si-containing minerals on goethite transformation in the Bayer digestion process, which could efficiently utilize the Fe- and Al-containing minerals present in goethitic bauxite. In this work, the interactions between anatase or kaolinite with goethite during various Bayer digestion processes were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The results showed that anatase and kaolinite hindered the transformation of goethite. Anatase exerted more significant effects than kaolinite due to the dense sodium titanate layer on the goethite surface after reacting with the sodium aluminate solution. Adding the reductant hydrazine hydrate could eliminate the retarding effect by inducing the transformation of goethite into magnetite. In this process, titanium was embedded into the magnetite lattice to form Ti-containing magnetite. Furthermore, the weakening of the interaction between magnetite and sodium aluminosilicate hydrate reduced the influence of kaolinite. As a validation of the above results, the reductive Bayer method resulted in the transformation of goethite into goethitic bauxite with 98.87% relative alumina digestion rate. The obtained red mud with 72.99wt% Fe 2 O 3 could be further utilized in the steel industry. This work provides a clear understanding of the transformative effects of Ti- and Si-containing minerals on iron mineral transformation and aids the comprehensive use of iron and aluminum in goethitic bauxite subjected to the reductive Bayer method.
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